Suprachiasmatic nuclei (SCN) constitute the parts of the hypothalamus that are involved in the control of more than 150 daily biological cycles, including cortisol and melatonin secretion, appetite, body temperature, vigilance and sleep rhythms.
Synchronization of suprachiasmatic nuclei with the 24-hour cycle generated by the earth's rotation is modulated by the alternating exposure to light and darkness. The response of SCN to light conditions is attributable to the presence of light-sensitive receptors in the retina, which signal the SCN accordingly. As natural light conditions vary through each 24-hour period, SCN receive different stimuli and modulate biological rhythms accordingly. As such, SCN were designated colloquially as the “biological clock”.
Modulation of the SCN activity and biological rhythms controlled thereby tend to be problematic where a subject's activities are not synchronized with the natural light and dark cycle. One known example relates to night-shift workers, where individuals are mentally and physically solicited while their circadian rhythms advocate for sleep. As such, the desynchronisation of work-related activities and natural light and dark cycle is associated with a decrease in vigilance, a decrease in productivity and an increase of work-related incidents.
Desynchronisation of human activities and natural light conditions can also be observed when traveling by jet airplane across several time zones, a condition best known as “jet lag”. While sporadic travellers can adapt to this situation relatively easily after a few days, jet lag may represent a major concern for commercial aircraft crews (e.g. pilots, flight attendants), which do not have the time to adapt and are therefore constantly submitted to circadian rhythms desynchronisation. Further, modulation of SCN activity also appears to influence mood, psychological wellness and light-associated psychological disorders such as, for instance, seasonal affective disorder, a condition also know as winter depression.
Several solutions have been attempted to alleviate problems associated with light-sensitive conditions or with desynchronisation of light cycle and human activities, including the use of artificial light. For instance, U.S. Pat. No. 6,554,439 discloses an apparatus that mimics the intensity and spectrum of natural light and other light dynamic conditions. The apparatus comprises a plurality of light sources of various colours, the light sources being controlled by a computer. The computer modulates the light sources to achieve desired light conditions to mimic natural light conditions (i.e. generally white light) and can be used to modulate circadian cycle and treat psychological disorders.
Similarly, U.S. Pat. No. 6,623,512 discloses a method and an apparatus for modulating circadian cycles or treating a seasonal affective disorder in an individual. The method includes the exposure of the individual's eye to flashes of white light, where each light flash has duration ranging between 1 and 500 milliseconds. According to this method, one light flash per minute is emitted and the individual is subjected to light treatment between 5 and 180 minutes. Because the individual can perceive the alternation of light flashes and light interruptions, refer to as the stroboscopic effect, the method taught in the U.S. Pat. No. 6,623,512 may lead to discomfort, especially when the subject is exposed to such light conditions for an extended period of time.
Further, because the methods and apparatuses taught in both previous patents make use of a generally white light (i.e. a blend of all colors in the spectrum of visible light), their efficiency tends to be reduced. It was indeed reported that the rods and cones (i.e. the classical photoreceptors capable of sensing the visible light spectrum) would not be essential to the transmission of light stimuli to the SCN, this function being rather accomplished by the recently discovered melanopsins, photoreceptors found in less than 1% of the total ganglion cell population.
Contrarily to rods and cones, sensitivity of melanopsins would be confined to a light spectrum in which wavelengths range from 420 to 540 nm, with a sensitivity or wavelength peak between 446 and 483 mm. This wavelength range encompasses the lights that are generally perceived as being blue (wavelength peak at about 470 nm) and green (wavelength peak at 525 nm). Blue light has been shown to be more efficient than white light with respect to the biological impact on performance, vigilance and general resynchronization of the biological clock. As such, use of a light consisting in a broader range of wavelengths, as disclosed in U.S. Pat. Nos. 6,554,439 and 6,623,512, tends to be purposeless and may ultimately lead to energy cost increases.
However, the use of blue light to modulate SCN response is not without limitation. For example, melanopsin photoreceptors tend to degrade fast and not to regenerate upon continuous exposure to blue light, which contribute to reduce the efficacy of blue light upon such sustained exposure.
But more importantly, blue light has been shown to be toxic for retina and to increase the risks of macular diseases, which side effects are commonly referred to as “blue light hazard”. Specifically, blue light is absorbed by lipofuscin, which in turns triggers the production of free radicals. Because the effects of blue light tend to be cumulative over the entire life of the subjects, it may cause irreversible damage.
It would therefore be profitable to be provided with a system that makes use of light for modulating a circadian rhythm, a vigilance state or a psychological condition, where the system allows overcoming the drawbacks generally associated with such use of light.